[entities] Disallow `requirement_level` field in the `identity` section

[dmitryax]

Is your feature request related to a problem? Please describe.

Per discussion with @jsuereth, all identifying attributes should be required. Therefore, having a requirement_level field in the identity section of the entity definition in semconv-syntax.v2.md#entities-definition creates confusion.

Describe the solution you'd like

Disallow the requirement_level field in the identity section of the entity definition in the v2 syntax and assume all identifying attributes are required.

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